1. Field of the Invention
This invention relates to self-pumped phase conjugate mirrors and to short pulse transient optical energy transfer systems.
2. Description of the Related Art
A phase conjugate mirror (PCM) produces a retroreflection of an incident beam, with the phase of the reflected beam reversed from that of the incident beam at the point of reflection. Several different methods of producing phase conjugated beams have been discussed in the literature, including four-wave mixing and self-pumped mechanisms. The theory and operation of PCMs is described in a chapter by Feinberg, "Optical Phase Conjugation in Photorefractive Materials", within the text "Optical Phase Conjugation", ed. by Fisher, Academic Press, Inc., 1983, pages 417-443.
Self-pumped PCMs form a phase conjugate of an input beam without the use of separate pump beams. They have numerous applications in fields such as optical data processing, laser sources and optical diagnostic systems. This elimination of separate pump beams offers considerable practical advantages over other types of conjugators.
Self-pumped PCMs have been developed based upon stimulated Brillouin or Raman scattering, and the photorefractive effect. Those employing Brillouin or Raman scattering are generally used with high power pulsed laser beams, such as from a Nd:YAG laser, but are not practical with low power continuously operated lasers such as HeNe or low flux argon ion laser devices. Self-pumped PCMs based upon the photorefractive effect use a photorefractive material with a high electro-optical coefficient as the phase conjugating medium. This type of self-pumped PCM has been employed with continuously operating, low power lasers. None of these devices work well for pulses on the order of picoseconds in duration, or in general for pulse trains in which the individual pulse durations are less than about 50 picoseconds (ps). SBS PCMs also introduce a small frequency offset on the phase conjugate wave that is undesirable in certain applications.
Self-pumped PCMs based upon the photorefractive effect are described in Feinberg, "Self-Pumped Continuous-Wave Phase Conjugator Using Internal Reflection", Optics Letters, Vol. 7, No. 10, October 1982, pages 486-488, and in Cronin-Golomb et al., "Theory and Applications of Four-Wave Mixing in Photorefractive Media", IEEE Journal of Quantum Electronics, Vol. QE-20, No. 1, January 1984, pages 12-29. Such devices have currently been demonstrated only with continuous wave (cw) and repetitively pulsed lasers, although it may be possible to employ the photorefractive effect to conjugate single nanosecond pulses. For pulses on the order of picoseconds, the photorefractive effect that has been observed is much too weak to produce self-pumped conjugation. In addition, self-pumped PCMs that use the photorefractive effect require materials such as BaTiO.sub.3 or strontium barium niobate that are relatively hard to obtain in small crystals, such as 5.times.5.times.5 cm.sup.3.
Outside of PCMs, another mechanism that has been observed to yield an energy transfer between optical beams is the transient energy transfer phenomenon. This differs from optical amplification by the photorefractive effect, which is produced by the shifted grating found in photorefractive materials when diffusion is the dominant transport mechanism, or is obtained by a frequency offset between two beams when drift is the dominant transport. The transient energy transfer phenomenon is believed to have been first publicly described by Vinetskii et al., "Transformation of Intensities and Phases of Light Beams by a Transient `Undisplaced` Holographic Grating", Sov. J. Quantum Electronics, Vol. 7, No. 2, February 1977, pages 230-233, and was further developed in Eichler et al., "Picosecond Pulse Amplification by Coherent Wave Mixing in Silicon", Physical Review, June 1, 1987, pages 4673-4678, and Dubard et al., "Beam Amplification by Transient Energy Transfer in GaAs and Si", Proceedings of the SPIE, Vol. 1017, No. 27, September 1988, pages 172-175.
Transient energy transfers can occur between beams in non-linear optical media when the beams are pulsed very rapidly, with pulse durations less than the grating response time (the time required for an optical grating to build up within the medium). If the pump and probe beam intensities are unequal, a transient energy transfer can occur from the pump to the probe beam if the response time of the medium's non-linearity is on the order of or longer than the pulse duration. Transient energy transfers have been realized using the free-carrier non-linearity, thermal gratings, and the photorefractive effect in LiNbO.sub.3. Transient energy transfers for picosecond pulses using the free-carrier non-linearity in GaAs and Si were reported in the Dubard et al. article mentioned above. A probe beam gain on the order of 15 resulting from a transient energy transfer from a separate pump beam was reported in this article.
While the transient energy transfer phenomenon is of interest, it has not addressed the limitations of currently available self-pumped PCMs described above in the area of very short duration pulses.